It may be the jewel of the Sierra, but when President Obama visits Lake Tahoe on Wednesday for a lakeside summit, his focus will be on the ways the mountain gem is being tarnished.

The annual Lake Tahoe Summit, attended by scientists, dignitaries and environmental activists, is both a celebration of the spectacular forested slopes surrounding the mesmerizing deep blue lake and a warning about what can happen if humans take such a place for granted.

The president’s speech at the outdoor arena at Harveys in Stateline, Nev., will highlight how the area’s veneer of natural beauty hides a host of ecological problems — warming water, alien species, pollution, algae — all of which are being exacerbated by global warming.

Tahoe’s problems are important, environmental scientists say, because this once-pristine ecosystem is steadily deteriorating. If a place like Tahoe is threatened, they say, no place where people live in America, no matter how beautiful, is safe.

“The president being here focuses attention on the fact that we still need help,” said Jesse Patterson, deputy director of the League to Save Lake Tahoe, also known as Keep Tahoe Blue. “Lake Tahoe is a national treasure. Once it is lost, you will never get it back.”

Jesse Patterson of the League to Save Lake Tahoe displays an invasive plant known as Eurasian watermilfoil that is clogging the Tahoe Keyes Marina.

Photo: Michael Macor, The Chronicle

Obama will be the first sitting president to attend the summit since Bill Clinton held a presidential forum there in 1997, kicking off the tradition. The summit has since become a yearly gathering of leaders dedicated to the goal of sustaining Lake Tahoe, and this year’s guests will include Gov. Jerry Brown and U.S. Sens. Dianne Feinstein and Barbara Boxer.

For many years, the biggest problem was lake clarity, which hit an all-time low in 1997. The second-deepest lake in the nation — behind Oregon’s Crater Lake — was so murky at that time that a disk lowered into the water could be seen only 64.1 feet down.

That was compared with 102.4 feet in 1968. Lake Tahoe was once so astonishingly clear that Mark Twain likened boating on it to floating on air.

How to watch

To view a live stream of the Lake Tahoe Summit at 1:30 p.m. Wednesday, go to bit.ly/2bzhSla

Read More

“Every little pebble was distinct, every speckled trout, every hand’s-breadth of sand,” Twain wrote in his semi-autobiographical novel, “Roughing It,” describing his view from a boat into Tahoe’s recesses. “Down through the transparency of these great depths, the water was not merely transparent, but dazzlingly, brilliantly so.”

The first summit led to a set of goals to improve conditions on the lake, including state and federal money for research. The Lake Tahoe Environmental Improvement Program was created with the initial aim of restoring visibility to a depth of 100 feet within 65 years.

More than 50 public and private organizations, including the Tahoe Regional Planning Agency and the Tahoe Conservancy, joined forces over the past two decades to address the problem. About $1.9 billion in public and private money has since been spent on hundreds of restoration projects around the lake, from new roads and drainage to runoff basins and more transit options.

Measures have been taken to reduce air pollution and prevent oil and gasoline from washing off roadways into the lake. New construction regulations, which also limit algae-causing runoff and other activities detrimental to the lake, have been put in place.

Jeff and Linda Soares of Santa Cruz relax on the beach of the Upper Truckee Marsh Restoration Project which is managed by the California Tahoe Conservancy, in Lake Tahoe, California as seen on Tuesday Aug. 30, 2016.

Photo: Michael Macor, The Chronicle

As a result, the steady decline in clarity appears to have been halted. The average depth at which a person could see a disk was 73.1 feet last year — a 4.8-foot decrease from the previous year but roughly in line with measurements over the past 15 years.

The clarity improvements serve as “a precedent that this investment does pay off,” said Geoffrey Schladow, director of the UC Davis Tahoe Environmental Research Center, which has done the bulk of the studies of the lake ecosystem.

But there are other problems, Schladow said. The average surface temperature of the lake hit 53.3 degrees in 2015, the warmest on record. Over the past four years, temperatures in the lake have risen 15 times faster than the long-term rate, according to a UC Davis report.

Meanwhile, more rain than snow fell in the Lake Tahoe basin in 2015. There were only 24 days last year in which average air temperatures were below freezing, the fewest on record.

The warming water hampers natural lake currents, which mix cold water at the bottom with surface water. The lack of mixing leads to algae growth and less oxygen at depth. The soupier water, in turn, seems to have helped 30 or so nonnative species, including Asian clams, trout, bass, crayfish, catfish, goldfish and bluegill, and a host of wetland grasses and weeds.

A recent study documented a dramatic decline over several decades in native fish linked to the emergence of largemouth bass, bluegill and catfish.

The fundamental problem, scientists say, is that every time one problem is addressed, many more crop up, most related to a dramatic climatic shift in the Sierra. While still fighting man-made pollution, conservationists are focusing more and more on the environmental problems caused by global warming.

“What is different this year is that we are seeing more aspects of the lake’s internal physics changing, and that is bound to alter the ecology,” Schladow said. “It is a graphic example of how climate change is an equal player with land use practices, which have for the past 50 years had the largest impact on the lake.”

Obama’s presence is welcome to those who have fought to protect the famous lake. That’s because federal money for Lake Tahoe has dried up over the past five years as congressional Republicans tightened the purse strings on environmental spending.

“If I had any hopes for the summit, it would be for similar investments in research and actions on the ground that would be able to mitigate what we believe are the likely impacts of climate change,” Schladow said. “I don’t think this is just about Tahoe. If, at the end of the day, we say we can’t preserve Tahoe, there is little chance that we are going to be able to save thousands of lakes, streams and wetlands in California and around the country.”

Peter Fimrite is The Chronicle’s lead science reporter, covering scientific research, the environment and the cosmos. His beat includes earthquake research, marine biology, wildfire science and space exploration. He also writes about the cannabis industry, outdoor adventure, Native American issues and the culture of the West. A former U.S. Forest Service firefighter, he has traveled extensively and covered a wide variety of issues during his career, including the Beijing Olympics, Hurricane Katrina, illegal American tourism in Cuba and a 40-day cross country car trip commemorating the history of automobile travel in America.